The present invention relates to overclad fiber optic couplers that exhibit low excess loss.
Overclad fiber optic couplers comprise an elongated matrix glass body through which optical waveguide fibers longitudinally extend. The diameter of the central region of the coupler is smaller than that of the ends of the coupler, whereby the fibers are more closely spaced and are of smaller diameter in the central region than they are at the ends of the unit.
Overclad couplers are usually formed by inserting into the bore of a glass tube at least a portion of each of a plurality of optical fibers so that the fiber portions occupy the midregion of the tube. The tube bore has funnels at each end to facilitate fiber insertion. The resultant coupler preform is evacuated, and its midregion is heated and collapsed onto the fibers. The central portion of the midregion is thereafter drawn down to that diameter and coupling length which is necessary to obtain the desired coupling between the fibers. Often, a sealant such as UV curable epoxy is then applied to the ends of the tube bore for stabilization of the coupled fibers. Couplers having various kinds of coupling characteristics, e.g. wavelength division multiplexer (WDM), achromatic, and the like, have been made by this process. See for example, U.S. Pat. Nos. 4,931,076, 4,979,972, 5,009,692 and 5,011,251.
When the tubing glass is collapsed around the fibers, the fibers are subjected to forces that bend them. Overclad coupler process reproducibility and excess loss are significantly impacted by this bending of the fibers and by the reproducibility of the fiber geometry in the coupler preform prior to the stretch step. Depending on the location of the fibers within the bore and upon such factors as twist, tension, and the like, during the tube collapse step, the resultant couplers can exhibit high variability and high excess loss.
For certain types of couplers, for example, 1.times.N couplers where N.gtoreq.2, the problem of fiber twist during the tube collapse step was remedied by employing a small diameter tube bore. See U.S. Pat. No. 4,979,972. The coupler is formed of standard single-mode fiber having an outside diameter of 125 .mu.m and a 250 .mu.m coating diameter. Coating is stripped from a central portion of a first fiber intermediate its ends, thus leaving two coated end portions. Coating is also stripped from an end portion of a second fiber. The diameter of the tube bore is made as small as possible so that the walls thereof will need to undergo a minimum of inward movement before total collapse onto the fibers is achieved. A bore size that is sufficiently larger than the coating diameter, for example 270 .mu.m, will permit one coated end portion of the first fiber to be threaded through the bore without coating material rubbing onto the surface of the bore. The stripped end of the second fiber can be inserted into the bore simultaneously with the stripped portion of the first fiber or after the stripped portion of the first fiber has been centered in the tube midregion. In either event, the stripped portions of the two fibers can be centered in the midregion of a tube having a bore that is only slightly larger than the combined diameter of the two stripped fiber portions. Devices having very low excess loss have been made by this technique.
However, as the coating diameter becomes larger than twice the fiber diameter, the tube bore diameter must be made larger in order to accommodate the coating. The larger diameter bore will adversely affect device excess loss.
A coupler having more than one fiber that is stripped in its central portion, such as a 2.times.2 coupler, also requires a relatively large bore. One fiber can be threaded through the bore until the stripped portion is centered in the tube midregion and the ends of the coated portions are located in the funnels. The second fiber is then threaded through the bore by first inserting one of its coated end portions. Thus, the bore diameter must be slightly larger than the combination of the fiber diameter plus the coating diameter. When using this combination of fibers and tube, fibers can easily become twisted during the tube collapse step, and the resultant device can therefore exhibit large excess loss.
One attempt at preventing fiber bending during the tube collapse step was to fill any excess space in the tube bore with spacer fibers composed of material which is the same as or similar to that of the tube. The idea is that the spacer fiber will prevent twisting or bending of the optical fibers by reducing space in the tube bore by creating a tighter fit. While this method will produce a high reliability coupler with low excess loss, it is cumbersome and not likely to be automated.
Fiber optic couplers having low polarization dependent loss can be made by employing a tube that has a relatively hard glass (high silica content) in the region adjacent the fibers. However, when tubes having conventionally shaped bores (for example, circular cross-section bores) are formed of such high silica content glass, the problem of fiber bending during tube collapse can be exacerbated, and excess loss can be increased.